Recently, a two-terminal organic electroluminescent (EL) device has been suggested as a self-emission display device, and thus an OELD panel or an OELD device employing the two-terminal organic EL device has been actively developed.
The typical two-terminal organic EL device is referred to as an organic light emitting diode (OLED). The OLED has a pair of electrodes, an anode and a cathode, together with an organic EL layer interposed between the anode and the cathode. The representative OLED has a multi-layered structure of anode/hole transport layer (HTL)/organic light-emitting layer (EML)/electron transport layer(ETL)/cathode, which was suggested by Tang et al. The OLED having the multi-layered structure of (HTL/EML/ETL) exhibits high luminescent efficiency and thus has been adopted in almost all kinds of current OELDs under the development. Further, a hole injection layer (HIL) may be further formed between the anode and HTL or an electron injection layer (EIL) may be further formed between ETL and the cathode.
When a voltage is applied to the OELD, the electrons from the cathode and the holes from the anode are injected into EML, and thereafter the recombination of the injected holes and electrons at EML excites the emitting centers, thereby emitting EL lights. In the present specification, all layers interposed between the anode and the cathode are referred to as an “EL layer”. Therefore, the EL layer can include the above-mentioned HIL, HTL, EML, ETL and EIL. Moreover, a single organic layer can also be used to form the EL layer. Also, luminescence of the EL device in the present specification is referred to as a “driving of the EL device”.
Driving system of the OELD includes a passive matrix type system and an active matrix type system. The passive matrix type OELD has an anode of transparent electrode in the form of stripe patterns on a transparent insulating substrate, an organic EL layer and a cathode of metal in the form of stripes perpendicular to the anode stripes, which are sequentially formed.
In the passive matrix type OELD, one of scanning lines of cathode (or anode) is selected and one or more data signal lines of anode (or cathode) among a plurality of data signal lines are selected, then, luminescence is generated from the pixels connected to the selected scanning line and the selected data signal lines. Data signal (or video signal) inputted from an outside is edited by a signal driving circuit, thereby transporting data signal to a signal line. Integrated circuit (IC) chip for the signal line driving circuit may be mounted on a display device in a manner of tape automated bonding (TAB) or may be directly bonded to a pixel substrate, thereby being assembled with the display device.
The active matrix type OELD includes a driving semiconductor element and a switching semiconductor element wherein those are prepared for each of a plurality of pixels arranged in the form of matrix type. The semiconductor elements are connected to one of the electrodes, the cathode and the anode, of the OELD, thereby controlling current flow through the OELD. Unit pixel of the active matrix type OELD includes an OLED and a transistor (or transistors) having a semiconductor film formed on an insulating substrate. Here, the transistor having a semiconductor film is hereinafter referred to as “a thin film transistor (TFT)”.
The active matrix type OELD has a plurality of OLEDs, a plurality of gate signal lines, a plurality of source signal lines, a plurality of power supply lines, a plurality of TFTs for driving and switching, and a plurality of capacitors, which are formed on the insulating substrate. Generally, at least two TFTs including at least one switching TFT and at least one driving TFT and one capacitor are provided for each of pixels in the active matrix-type OELD.
As described above, the OLED is a self-emitting device having high brightness and it does not require a backlight source, so the thickness and weight of the OLED display can be relatively reduced in comparison with a conventional liquid crystal display (LCD). By the above-mentioned reasons, it is a trend that the OELD is used as a display section of a portable information terminal such as a mobile computer, a mobile telephone, a mobile game player or an electronic book etc., instead of the LCD. The development of the OELD having a higher brightness of luminance under a lower operating voltage and having high production yield to accommodate the present trend, has been continuously required.